Frequency changing system



Sept. 29, 1936. F: TERMAN 2,055,736

- FREQUENCY CHANGING SYSTEM Filed Oct. 20. 1930 I INVENTORT as I FREDERICK E. TERMAN.

ATTORNEY Patented Sept. 29, 1936 UNITED STATES PATENT OFFICE 3 Claims.

My invention relates to modulating or frequency changing systems, and particularly to a system for modulating wherein no carrier frequency current is developed.

It has long been known that where currents of two different frequencies are mixed or combined, and applied to a device having a non-linear response, there will be present in the output of the device both of the original frequencies, together with frequencies representing the sum and the difference of the original frequencies. Generally accepted nomenclature terms the lower of the original frequencies the modulating frequency, and the higher of these frequencies the carrier frequency. The sum frequency and the difference frequency are known as side-band frequencies.

The sum of the three last frequencies mentioned may be represented as a single frequency, that of the carrier, of varying amplitude, and is what is usually referred to as a modulated wave, particularly where the modulating frequency is of much lower order than the carrier frequency.

Where both modulating and carrier frequencies are high, and there is a relatively small difference between them, the lower side-band frequency is often referred to as the beat frequency, little attention being paid to the carrier and the sum or higher side-band frequencies in this instance, but the process of generating such a beat frequency is essentially one of modulation, and the term side-band frequency is to be preferred, and is the one which will be used throughout this specification.

Various systems have been used for suppressing one or more of the frequencies present in the output, these systems being of greater or less complexity. Where it is desired to suppress the carrier, the system used in the past has involved careful balancing of circuits and complex apparatus. Where other frequencies were to be suppressed, other systems have been used, most of these being essentially simple filter systems.

Among the objects of my invention are: first, to provide a frequency changing system requiring no non-linear elements; second, to provide a frequency changing or modulating system wherein the carrier frequency is entirely absent from the output, and this without the use of balancing methods; third, to provide a frequency changing system which is equally applicable to high or low frequency modulating or carrier waves and to the production of high or low side-band frequencies; and fourth, to provide a new type of beat frequency receiver for continuous radio waves.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a schematic circuit diagram illustrating the fundamental principle of my invention.

Figure 2 shows a family of curves representing the characteristics of one form of variable admittance.

Figure 3 is a sechematic representation of a practical circuit embodying my invention. 15

Figure 4 is a diagram showing a modification of Figure 3.

Figure 5 is a circuit diagram illustrating a mod ified form of my invention.

Figure 6 is a circuit diagram showing the connections of a dynatron used as a variable admittance element; and

Figures '7 and 8 are circuit diagrams of beat frequency radio receivers embodying my invention.

The frequency changer of my invention comprises essentially a circuit element having an admittance which varies as a function of the carrier frequency, and to which a modulating frequency is supplied. Under these circumstances, no current or voltage of the carrier frequency will appear in the circuit, but there will appear a current and/or voltage of each of the two side-band frequencies, which may be utilized in the manner to be described below.

Figure 1 shows the bare essentials of my invention, comprising a source I of modulating frequency voltage, connected to a variable impedance element 2. For reasons of mathematical simplicity, this impedance will be defined in terms 0 of its reciprocal, i. e., its admittance Y which we will take equal to Yu+y sin wt, where wt equals 21r times the carrier frequency fc. If the source I generates a voltage ES=E sin ot, a current will flow in the circuit which may be expressed by the 45 equation:

I :YoE sin vt+yE sin ct sin wt This equation may be expanded into the form:

band frequencies. It will be seen that no current of carrier frequency is present in the circuit.

The application of this arrangement in modulating a low signal frequency upon a high frequency carrier is illustrated in Figure 3. The signal frequency generator I is connected in series with a circuit tuned to the carrier frequency (or if desired, to one of the side-band frequencies) and comprising a condenser 3 and an inductor 5 in series with the variable impedance element 2. A high frequency choke coil 6 is preferably shunted around the condenser 3, so that the circuit as a whole will offer low impedance to the signal frequency. The voltages of the side-band frequencies will be developed across the condenser 3 and the inductor 5.

A slightly diiferent arrangement is shown in Figure 4, wherein the variable impedance element 2' is connected across the condenser 3 instead of in series with the circuit, the other elements in the circuit having the same relation as in Figure 3, and carrying the same reference characters.

Still another modification is shown in Figure 5. In this arrangement the generator I supplies a circuit comprising an impedance ill so large that the current flowing is substantially unaifected by the variations in the value of the element ll, whose impedance Z equals ZO-l-Z sin wt. In this case, the side-band voltages are developed across the element l l, and may be applied to the grid of a vacuum tube or to other means of utilization.

Thus far, the nature of the variable impedance element has not been discussed, but there are a number of ways in which it may be achieved. Thus, for moderate frequencies, it may comprise a variable condenser or a variometer rotated by a suitable form of motor at a speed to give the required frequency. For extremely high frequencies this is not practical, and therefore I prefer to use a vacuum tube arrangement which operates as a variable conductance.

This is an adaptation of the so-called dynatron, wherein the signal frequency generator I (Figure 6) is connected to the plate E5 of a fourelement vacuum tube IS. The other side of the generator I connects through a series condenser l1 and inductor l8, tuned to the carrier frequency, to a positive tap on the battery or other voltage source 20. The most positive point on the battery connects with the shield grid 2| of the dynatron, while the negative end of the battery connects with the cathode 22. A voltage of carrier frequency is impressed across the control electrode 23 and the filament 22 by a suitable oscillator 25.

With a suitable constant voltage impressed upon the screen grid, varying values of control electrode and plate voltage will cause varying current to fiow in the plate circuit as shown by the curves of Figure 2, wherein each curve of the family represents the current flowing with varying plate voltages at a particular control electrode voltage. In the arrangement shown, the plate of the tub-e operates as an emitter of secondary electrons, and the total current flowing in the circuit is equal to the difference between the number of primary electrons striking the plate and the number of secondary electrons leaving it. It will be seen that all of the curves of the family intersect at a point corresponding to some definite value of plate voltage E0, and for this value of plate voltage no current flows. If the dynatron be adjusted to this particular value of plate voltage, and a sinusoidal voltage EC sin at be applied to the control electrode, the dynatron becomes an element having a linear admittance characteristic, the admittance varying in accordance with the equation Y=G sin wt. Mathematically, the admittance of the device is a pure conductance of negative sign and in general the circuit should contain a sufiicient positive impedance so that the negative characteristic will not cause self-oscillation.

Other variable admittance elements are described in my co-pending applications, Serial Numbers 539,655 and 539,656 filed May 25, 1931, the latter patented on March 13, 1934, #1,950,759.

As is the case in Figures 3 and 4, the condenser ll is shunted by a high frequency choke 26 so that the impedance of the circuit to the modulating frequency may be low.

It will be noted from the curves of Figure 2 that there are two points where the family of curves intersect, the second point being that corresponding to zero plat-e voltage. Modulation of the type herein described will also occur if the variable tap ll? be adjusted to give zero or negative voltage upon the plate. In this case, however, the modulation is non-linear, and a series of harmonic frequencies will appear in the output of the device, i. e., across the condenser l! as indicated by the leads 2? and 28. These leads may be connected to a suitable form of amplifier, or the side-band frequencies may be used di rectly. Under circumstances where distortion is not objectionable, this modification is advantageous because of its simplicity.

It is not essential for the generation of sideband frequencies that the variable elements change in value of admittance in direct proportion to the sine of the carrier frequency. Any periodic variation of admittance with frequency will cause the generation of the two side-bands, non-sinusoidal variations of admittance causing the appearance of harmonic frequencies which may or may not be deleterious. Thus, as was indicated in connection With the discussion of Figure 5, if the total impedance of the circuit be high the variation of the modulating elements may be in proportion to a sine function of impedance instead of admittance. Such a variation gives increasing amount of harmonic distortion as the value of the non-varying component of the impedance becomes lower.

In Figure '7 is shown the adaptation of the modulating system of my invention to a beat frequency radio receiver. The receiver shown comprises an antenna 35 coupled through a few turns of the inductance 3! to a resonant circuit comprising this inductance and the variable condenser 32, the lower end of the inductance being connected to the ground 33. The variable impedance element comprising the dynatron 35, connected across the condenser 32, is supplied from the local oscillator 36 with a carrier frequency differing from the frequency which it is desired to receive by a relatively low amount, preferably within the audio frequency range. The connection from the oscillating circuit to the anode of the dynatron is through a radio frequency by-pass condenser 31 around which is shunted the primary 38 of a transformer, the secondary 39 of which may be connected to the desired type of amplifier.

The signal frequency and the upper side-band frequency are shunted around the transformer by the condenser Bl, but the lower side-band frequency is transferred through the transformer to be utilized as desired.

A slightly different form of beat frequency receiver operating in accordance with my invention is shown in Figure 8. Here the antenna is connected across the condenser 4| which forms a resonant circuit in connection with the inductance 42 and radio frequency by-pass condenser 43. The primary 45 of the low frequency transformer is connected across the condenser as in the previous case. The tuning condenser 4| is shunted by the radio frequency choke 46, in order to supply a low impedance path for the side-band frequency. In this instance the variable impedance element 41 is not directly in series with the modulating. frequency circuit, but is connected across the coil 48 which is inductively coupled to the coil 42. It will, therefore, be seen that in so far as the modulating frequency circuit is concerned, the variable impedance element is the coil 42, since this is the primary of a transformer and the effective impedance of a transformer primary varies with the impedance in series with its secondary. The effeet is, therefore, the same as though the signal frequency circuit actually had the variable imanode, and a control electrode, in a circuit connected to the secondary emitting electrode and the anode, which comprises the steps of adjusting the secondary emitting electrode current of said dynatron to zero, applying a modulating voltage in said circuit, and applying a carrier frequency voltage to the control electrode of said dynatron.

2. The method of operating a dynatron having a cathode, a secondary emitting electrode, an anode, and a control electrode, in a circuit connected to the secondary emitting electrode and the anode, which comprises the steps of adjusting the secondary emitting electrode current of said dynatron to zero, applying a modulating voltage in said circuit, applying a carrier frequency voltage to the control electrode of said dynatron, and withdrawing energy of side-band frequency from said circuit.

3. In combination with a source of modulating frequency current and a generator of carrier frequency potential, a circuit including said modulating frequency source, a vacuum tube having a control electrode connected to said carrier frequency generator and a secondary emitting electrode connected in said circuit, said secondary emitting electrode being adjustable to carry zero current at zero potential of said source and the admittance of said circuit varying with the potential of said control electrode; and means for withdrawing a side-band frequency from said circuit.

FREDERICK E. TERMAN. 

